This invention relates to ultrasonic transducers and more particularly to an ultrasonic transducer in the form of a crossed linear array for ultrasonic medical imaging and for biopsy needle guidance.
In a linear array ultrasonic scanner the transducer consists of a series of individually addressable piezoelectric segments which emit ultrasonic waves when energized by the application of electricity. In a linear array the individual segments of the transducer are activated sequentially from one end to the other to generate a rectangular image. The ultrasonic waves emitted by the transducer are focused by the activating electronics and by an acoustic lens disposed between the transducer and the tissue to be imaged. The ultrasonic image generated by a linear array transducer is that of a rectangular "slice" of the tissue.
However, the ultrasonic image generated is only two dimensional. That is, the image has no usable thickness information. Accordingly, the use of a linear array scan to place, for example, a biopsy needle is at best difficult because of the lack of usable thickness information. If the needle is not precisely in the plane of scan the needle will be invisible to the physician. Furthermore, under certain circumstances even if the needle is visible it may still be improperly placed. Accordingly, it is desirable that a physician be able to determine in three dimensions where a biopsy needle has been placed.
In ultrasonic medical imaging another common type of scanning mode used with a segmented transducer is the so called "phased array" system. In a phased array the activating electronics are used to simulate the scanned motion of a transducer without actually physically moving the transducer. In a phased array system the segments of the transducer are activated such that the segments of certain sections of the transducer are delayed with respect to the others. Such a device produces a wedge shaped "sector scan" image. However in the sector scan the image formed is again only two dimensional with no useful thickness information.
A transducer of the phased array typed suitable for ultrasonic imaging in more than one plane is shown in U.S. Pat. No. 4,671,293 which issued to the inventor herein and shows a phased array two plane transducer. However, the electronics needed to drive a phased array transducer are considerably more complex than that of a linear array. Accordingly, an improved two plane linear array device suitable for imaging in two planes to permit accurate biopsy needle placement is desired. The present invention is directed to providing such a device.
U.S. Pat. No. 4,570,488 is directed to a various two dimensional arrangement of ultrasonic linear arrays. In one embodiment, two perpendicular linear arrays overlap each other. In another example, two perpendicular linear arrays are arranged across each other. However at the intersection of the linear arrays the elements of the transducer are divided into a large number of small elements which form a two dimensional array. Such an array is disadvantageous because a large number of electrical connections to the individual elements of the two dimensional array are required at the crossing area. Furthermore, the electronics required to drive the array at the center so as to provide two orthogonal linear scans are also quite complex.